1. Field
This application relates generally to prosthetic devices and methods for restoring sight to the blind.
2. Description of Related Art
Among the many causes of blindness, retinitis pigmentosa (RP) and age-related macular degeneration (AMD) are prevalent, causing catastrophic damage to the photoreceptor layer within the retina. Recent studies have demonstrated, however, that the inner layers of the retina remain for the most part intact even many years after the onset of the disease and the cessation of useful visual acuity. Furthermore, it has been shown that local electrical stimulation of the output ganglion cell layer leads to viable visual percepts, suggesting the possibility of developing retinal prostheses and restoring vision. Based on these findings, both epiretinal and subretinal microstimulator arrays have been developed to provide retinotopic stimulation. Two approaches have been used previously to supply images of the external world to the microstimulator array: (1) the incorporation of light sensitive elements within the microstimulator array itself, and (2) the use of an external camera, mounted for example on a pair of eyeglasses.
The incorporation of photosensitive elements within the microstimulator array itself has the apparent advantage of using the existing corneal lens and crystalline lens for the formation of the optical image. The incorporation of photosensitive elements within microstimulator arrays is described in A. Y. Chow, V. Y. Chow, M. T. Pardue, G. A. Peyman, C. Liang, J. I. Pearlman, and N. S. Peachey, “The Semiconductor-Based Microphotodiode Array Artificial Silicon Retina”, IEEE International Conference on Systems, Man, and Cybernetics, 1999, 4, 12-15, 404-408, (1999) and F. Gekeler, H. Schwahn, A. Stett, K. Kohler, and E. Zrenner, “Subretinal Microphotodiodes to Replace Photoreceptor-Function: A Review of the Current State” Les Seminaires Ophtalmologiques d'IPSEN, 12, 77-95, (2001).
However, the implementation of such photosensitive elements within the microstimulator array has proven to be problematic for a number of reasons. The retina itself is a curved surface, and the optics of the eye are designed to focus onto this curved (Petzval) surface. Hence, implantation of a microstimulator array with photosensitive elements incorporated on a planar substrate, such as is typical of most semiconductor devices and very large scale integrated (VLSI) circuits, becomes increasingly difficult as the size of the microstimulator array increases. Second, the retinal tissue itself is fragile, and is easily damaged by the proximal implantation of devices with hard edges; again as is characteristic of most semiconductor devices and very large scale integrated (VLSI) circuits. Fabricating a microstimulator array with incorporated photosensitive elements on a pliable or curved substrate is problematic because most semiconductor materials capable of supporting photosensitive elements, and of also supporting necessary circuitry for biasing and gain, and neither pliable nor curved. Third, the incorporation of photosensitive elements without associated amplification is not capable of providing signals that are directly appropriate for localized electrical stimulation of the inner or outer layers of the retina. Fourth, the incorporation of photosensitive elements within the microstimulator array itself requires a tradeoff of space (area on the supporting substrate) between the photosensitive elements and any associated circuitry, on the one hand, and the stimulation electrodes and any associated circuitry and electrical connections (interconnection wiring, or metal traces) on the other hand. As microstimulator arrays are scaled up to higher and higher densities, the available space is increasingly required for the stimulation electrodes and interconnection wiring, leaving little if any space for localized photosensitive elements. Fifth, the incorporation of photosensitive elements (and any associated amplifiers, transformers, or signal conditioners) within the microstimulator array places an additional source of heat dissipation directly in contact with the thermally-sensitive retina itself, whether the microstimulator array is implanted subretinally or epiretinally. Sixth, the provision of electrical power for signal amplification at the surface of the retina is challenging, requiring an additional wired or wireless interconnection to the microstimulator array from the power source. Finally, the incorporation of photosensitive elements within the microstimulator array makes the implementation of post-image-acquisition but pre-stimulation image processing functions problematic, as either additional power consumptive circuitry must be added to the microstimulator array to perform these functions, or a wired or wireless interconnection must be provided to an ancillary device within which such processing is performed.
Additionally, the use of an external camera for capturing images of the external world would require implanted patients to employ (at times rapid) head motion to search the visual field or track moving objects, which can in turn lead to disorientation, dizziness, and nausea. Furthermore, the natural tendency of patients to foveate to the most visually interesting or important part of a scene would be to no avail with an externally-mounted camera coupled to the (internal) microstimulator array.
Miniature cameras have been developed for a wide range of applications, including surveillance, automated inspection, inspection in harsh environments, and certain biomedical applications. Such miniature cameras are not amenable to implantation within the human eye, as they are not designed specifically to work in conjunction with the biological corneal lens to comprise a two lens system that can form appropriate images of the external world on an image sensor array. In addition, they are too large to fit within the confines of the human eye, much less the confines of a supportive membrane such as the crystalline lens sac. The size of such cameras has been limited in large part by the difficulty of designing very short focal length lenses in the range of 1 to 3 mm with acceptable optical imaging performance. Miniature cameras developed to date have too high a mass to be supported within the human eye in general, and within the crystalline lens sac in particular, especially considering the needs for chronic implantation and rapid foveation. Such miniature cameras are also too power consumptive, which would lead to unacceptable temperature rises within the biological space surrounding an implanted intraocular camera.